1. Field of the Invention
The present invention relates to a method of improving the adhesion of a poly(ester-amide) hot melt adhesive composition, to a metal substrate and to metal articles filled or coated with the hot melt adhesive composition. More particularly, it refers to a method of improving the adhesion of a block copoly(ester-amide) filled with a finely divided metal powder to metal substrates by means of a tie coat of melamine formaldehyde resin, and to metal articles primed with a melamine formaldehyde resin and filled or coated with the block copoly(ester-amide) composition.
2. Description of the Prior Art
Hot melt adhesives are well known in the prior art. These materials are conveniently applied to a substrate in the molten state and upon cooling form an adhesive bond. However, a deficiency common to most of the hot melt adhesives of the prior art is their tendency to soften and flow at elevated temperatures, as for example, 70.degree. to 100.degree. C. with a resulting loss of bond strength. Consequently, these materials are not suitable for use over a broad temperature range.
Attempts to upgrade the softening and flow temperatures have involved using very high molecular weight resinous materials and/or crosslinking of the resin. These methods have resulted in materials with higher softening points and flow temperatures. However, in most cases the resulting material was not adapted to thermal processing because of its higher molecular weight and/or crosslinked structure resulting in extremely high application viscosity. Thus, these materials were not suitable for use as hot melt adhesives.
U.S. Pat. No. 3,650,999 discloses a poly(ester-amide) resin having improved adhesion and high temperature performance obtained by reacting a crystalline polyester, a C.sub.18 to C.sub.54 polycarboxylic acid and a primary diamine. However, this poly(ester-amide) in common with other hot melt adhesives has deficiencies in creep resistance at temperatures above 150.degree. C. in the range up to 205.degree. C. and above and in shrinkage when the hot melt is cooled to room temperature after application. Improvement in creep resistance can be obtained by incorporating inorganic particulate fillers.
In the manufacture and repair of metal bodies such as automobiles and appliances, solder compositions containing lead are frequently used to fill cavities and voids. These lead solders are a health hazard which mandates special handling to protect workers and are also extremely dense. Conventional hot melt adhesives are not satisfactory for such cavity and void filling applications because they cannot be sanded rapidly at assembly line speed, they do not readily accept paint because they bleed through, and they do not withstand the curing temperatures for the paint. Curable adhesives such as epoxies are generally unsatisfactory because they require careful metering of the components to provide good physical properties and bond strength, and because they take too long to cure to a sandable state.
A need therefore exists for a cavity or void forming composition which is less dense and toxic than lead solder, forms a strong bond to metal substrate, withstands extremes of humidity and temperature, is readily applied and rapidly sets to a sandable state, is easily sanded smooth, and accepts paint without bleeding through. These needs are met by the adhesive compositions disclosed in U.S. applications Ser. No. 654,490 and 654,679 filed Feb. 2, 1976.
The adhesive compositions disclosed therein comprise from about 70 to about 30 parts by weight of a poly(ester-amide) block copolymer melting in the range of about 155.degree. to about 225.degree. C., having from about 30 to about 70 percent by weight of crystalline polyester segments and from about 70 to about 30 percent by weight of amorphous polyamide segments and from about 30 to about 70 parts by weight of finely divided spheroidal metal powder selected from the group consisting of aluminum, iron, mild steel, stainless steel and zinc.
However, when filled adhesive compositions containing inorganic particulate fillers especially those containing high ratios of filler and/or block polyesteramide of high glass transition temperature are applied to bare metal substrates they tend to exhibit low peel strength at low temperatures particularly at temperatures below 0.degree. F. (-18.degree. C.). A need therefore exists for a method of improving the adhesion of the filled adhesive compositions to metal surfaces and enhancing the peel strength of the bond formed between the filled adhesive compositions and such metal substrates. A need also exists for metal articles coated or filled with such block copoly(ester-amide) compositions which exhibit improved bonding between the metal substrate and the block copolyesteramide. A need also exists for a primer or tie-coat which enhances the adhesion of the block copoly(ester-amide) to metal surfaces and which can withstand the high temperature which the metal is subjected to, prior to application of the hot melt adhesive. A need also exists for a primer or tie-coat which can be applied in essentially undiluted form free of toxic or flammable solvent.